While having a plaid (a fly plaid or any other kind) and a waistcoat from exactly the same material as the kilt they go with will make for a very handsome outfit, my understanding of Highland dress tradition is that this is not essential.
For example, the Atholl Highlanders parade wearing kilts and plaids that quite noticeably are not from the same dye lots.
And a waistcoat in the same tartan as the kilt, but in a different colour style, is quite acceptable. In fact, I have it in mind to order a waistcoat in Lindsay Muted to go with the Lindsay Modern kilt that I hope to be able to wear by Christmas.
With regard to black and brown leather, South African men generally have both black and brown shoes, and to wear black on more formal occasions. This may have something to do with the brown boots worn by the South African Army. As I have mentioned before, South Africa’s tartan-wearing regiments wear black boots as an exception to the rule.
I tend to wear brown shoes with my brown leather day sporran, and black shoes with my formal sporran.
Sir Robert may, of course, do as he pleases in this regard.
As to headgear, I find that formal occasions often coincide with cold weather, and I must wear a hat outdoors when it is cold. But formal events are generally indoor, so one could wear a bonnet going to the event and take it off once inside. A glengarry is perhaps more suitable for this purpose, since it folds flat, but I will stick with my tam o’shanter.
Regards,
Mike